P010010-00-R - LIGO - California Institute of Technology

1.3 Interferometer Sensitivity
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Concurrent with the construction of the first generation LIGO detector (LIGO I) is the
work of an international collaboration, the LIGO Scientific Collaboration (LSC), to
develop the next generation interferometer, LIGO II.[12] The planned improvements
lead to nearly an order of magnitude increase in the sensitivity across the bandwidth,
as well as extending the bandwidth to lower frequencies. To understand how this is
to be accomplished requires some description of the limits to the sensitivity of the
detector, and how that sensitivity is shaped. In brief, the sensitivity is defined by the
noise which limits the measurement (seismic, thermal, and shot noise), as well as the
gain of the interferometer. LIGO II will use improved technology and materials to
reduce the noise due to seismic and thermal noise. Increased laser power will reduce
shot noise. An advanced interferometer configuration will shift the maximum gain of
the detector away from DC into the bandwidth of interest by placing a mirror at the
output, similar to the original GEO600 interferometer. This will generally be referred
to as a signal tuned interferometer.
1.3.1 Seismic Noise
The measurement of the gravitational wave is accomplished by monitoring the relative
distance between the surfaces of two test masses. Any other force which disturbs the
center of mass or the surface of the mass itself makes the measurement ambiguous – it
is unclear whether the disturbance is due to the gravitational wave or the noise force.
A ground-based interferometer must be, at some level, mechanically coupled to the
earth, hence the masses are subject to seismically driven vibrations. The dominant
part of the seismic power spectrum is at low frequencies. This varies, sometimes by
an order of magnitude or more around the world, but a moderately quiet site will
have a spectrum of roughly x(f) = 10 −8 m/ √ Hz × (1Hz/f) 2 .
Techniques for reducing the effects of seismic noise have long been known, and yet
new designs and approaches continue to improve vibration isolation. Two different
approaches have been taken: passive and active.